The invention concerns a method for the open-loop and closed-loop control of an internal combustion engine.
In an internal combustion engine with a common rail system, the quality of combustion is critically determined by the pressure level in the rail. Therefore, in order to stay within legally prescribed emission limits, the rail pressure is automatically controlled. A closed-loop rail pressure control system typically comprises a comparison point for determining a control deviation, a pressure controller for computing a control signal, the controlled system, and a software filter for computing the actual rail pressure in the feedback path. The control deviation is computed as the difference between a set rail pressure and the actual rail pressure. The controlled system comprises the pressure regulator, the rail, and the injectors for injecting the fuel into the combustion chambers of the internal combustion engine.
DE 197 31 995 A1 discloses a common rail system with closed-loop pressure control, in which the pressure controller is equipped with various controller parameters. The various controller parameters are intended to make the automatic pressure control more stable. The pressure controller then uses the controller parameters to compute the control signal for a pressure control valve, by which the fuel drain-off from the rail into the fuel tank is set. Consequently, the pressure control valve is arranged on the high-pressure side of the common rail system. This source also discloses an electric pre-feed pump or a controllable high-pressure pump as alternative measures for automatic pressure control.
DE 103 30 466 B3 also describes a common rail system with closed-loop pressure control, in which, however, the pressure controller acts on a suction throttle by means of a control signal. The suction throttle in turn sets the admission cross section to the high-pressure pump. Consequently, the suction throttle is arranged on the low-pressure side of the common rail system. This common rail system can be supplemented by a passive pressure control valve as a protective measure against excessively high rail pressure. The fuel is then redirected from the rail into the fuel tank via the opened pressure control valve. A similar common rail system is known from DE 10 206 040 441 B3.
Control leakage and constant leakage occur in a common rail system as a result of design factors. Control leakage occurs when the injector is being electrically activated, i.e., for the duration of the injection. Therefore, the control leakage decreases with decreasing injection time. Constant leakage is always present, i.e., even when the injector is not activated. This is also caused by part tolerances. Since the constant leakage increases with increasing rail pressure and decreases with falling rail pressure, the pressure fluctuations in the rail are damped. In the case of control leakage, on the other hand, the opposite behavior is seen. If the rail pressure rises, the injection time is shortened to produce a constant injection quantity, which leads to decreasing control leakage. If the rail pressure drops, the injection time is correspondingly increased, which leads to increasing control leakage. Consequently, control leakage leads to intensification of the pressure fluctuations in the rail. Control leakage and constant leakage represent a loss volume flow, which is pumped and compressed by the high-pressure pump. However, this loss volume flow means that the high-pressure pump must be designed larger than necessary. In addition, some of the motive energy of the high-pressure pump is converted to heat, which in turn causes heating of the fuel and reduced efficiency of the internal combustion energy.
In present practice, to reduce the constant leakage, the parts are cast together. However, a reduction of the constant leakage has the disadvantages that the stability behavior of the common rail system deteriorates and that automatic pressure control becomes more difficult. This becomes clear in the low-load range, because here the injection quantity, i.e., the removed fuel volume, is very small. This also becomes clear in a load reduction from 100% to 0%, since here the injection quantity is reduced to zero, and therefore the rail pressure is only slowly reduced again. This in turn results in a long correction time.